In a 3rd generation telecommunication system including the feature of High-Speed Downlink Packet Access (HSDPA), e.g. as described in the document 3GPP TR 25.848: “Physical Layer Aspects of UTRA High Speed Downlink Packet Access” issued by the 3rd Generation partnership Project (3GPP), every user equipment (UE) is allocated a dedicated channel (DPCH) in both directions to exchange higher layer signalling information between, e.g., user equipment and Radio Network Controller (RNC). Especially at high load, with a large number of HSDPA users in a cell, it will be required to allocate a large number of OVSF codes in the downlink. OVSF codes are Orthogonal Variable Spreading Factor codes. Therefore, in order to efficiently use the resources and save downlink OVSF codes the concept of a Fractional Dedicated Physical Channel (F-DPCH) has been introduced in Release 6 of the 3GPP specifications for 3rd generation telecommunication systems. The F-DPCH carries only TPC commands of several HSDPA users. This means the dedicated channels (TPC) of several user equipments are time multiplexed on one OVSF code in order to run power control. The corresponding signalling radio bearers (SRB) carrying the radio resource control (RRC) related information when using F-DPCH, said information relating, e.g., to cell change, active set update, RB reconfiguration, etc., are mapped on the HS-DSCH which thus leads to an efficient usage of the downlink channelization codes.
When F-DPCH is used, the signalling radio bearers are sent to the user equipment on the High-Speed Downlink Shared Channel (HS-DSCH). However, this channel does not support soft handover (SHO). This can in some cases lead to the situation that the signalling radio bearer can be lost due to poor HS-DSCH reception quality at the user equipment. The uplink quality, e.g. for the UL SRB:s, on the other hand can still be sufficient since both DCH and E-DCH can gain from the benefits of the soft handover operation. As apparent, e.g., from the document 3GPP TS25.331: “Radio Resource Control Protocol Specification (FDD)”, the signalling radio bearers (SRB) can carry vital radio resource management (RRM) related information such as, e.g., active set update, cell change, radio bearer reconfiguration, etc. This means that the loss of a downlink signalling radio bearer (SRB) can lead to unnecessary SRB retransmissions, call drops and increased delay in scheduling the user in the new HS-DSCH serving cell.
There are two previous solutions known to solve this problem:
According to a first prior-art solution, it is the user equipment that detects that the HS-DSCH reception quality is under a certain threshold value and performs autonomously a cell update. However, this solution implies the disadvantage that the user equipment does not know the best cell in terms of available radio resources. The user equipment may thus encounter the same problem in the new cell. As there are no possibilities for any kind of load control, this solution may even turn the problem to the worse. The Serving RNC (SRNC) also needs to do the pre-configuration of the user equipment for all the cells in the active set. Furthermore, re-routing of the data by the SRNC from the old serving cell to the new serving cell cannot be done until the user equipment has successfully informed the SRNC in order to switch to the new cell.
According to another prior-art solution it is the Node B that detects that a signalling radio bearer is not successfully transmitted to the user equipment and informs the SRNC to take appropriate measures. However, it may not be easy for the Node B to detect whether the signalling radio bearer has been lost or not. Further, the Node B is not aware of what type of information is carried by the signalling radio bearer, which implies that the Node B will indicate to the SRNC each time an SRB is lost and, consequently, would unnecessarily increase the Iub-signalling load. Finally, this proposed solution may involve long delays during which the call can be lost.